Cytometry is a technical specialty concerned with the counting and characterization of biological cells. FIG. 1 shows a simplified diagram of one technique known as flow cytometry. In a basic form of flow cytometry, cells 101 are suspended in a fluid and entrained single-file in a narrow transparent tube 102. The entrainment can be accomplished by any of several methods, including hydrodynamic focusing. A light source 103 illuminates each cell 101 as it passes a measurement location 104. Light source 103 may be, for example, a laser. Light from light source 103 is scattered by the cell 101 being measured. Some light 105 is scattered generally in the same direction as it traveled to reach the cell 101. Light 105 is sometimes called “forward scatter”, and may be collected by a forward sensor 106. Some light may be scattered in other directions as well. This light may be called “side scatter”, and some of the side scattered light 107 may be collected by one or more other sensors 108. Output signals from sensors 106 and 108 are sent to a computer 109, which may store and analyze the signals. By analyzing the amount and distribution of the scattered light, it is possible to discern information about each cell, for example its size and some information about its internal structure.
Flow cytometry may measure the scattered light directly, or may make use of fluorescence. In fluorescence cytometry, the cells may be marked with one or more fluorophores, which are excited by light from source 103 to produce light by fluorescence. The nature of the emitted light may reveal additional information about the cells.
The technique shown in FIG. 1 relies entirely on measurements of scattered light to infer information about the cell structure, but does not produce an image of any particular cell. In another technique, called “image cytometry”, an image of an individual cell may be recorded by a camera or microscope.